The invention relates to the exhaust-air guide of a fuel cell stack, in particular in a motor vehicle, having a cooling device, which belongs to the functional environment of the fuel cell stack, in the form of a cooler structure through which ambient air flows. With regard to the prior art, reference is made by way of example to DE 10 2008 029 529 A1.
Fuel cells, at least those of PEM type of construction, must be cooled during operation. For this reason, a suitable cooling device must be provided for a so-called fuel cell stack formed by a stack of multiple individual fuel cells. Such a cooling device is normally formed by a circuit for a heat carrier medium and a heat exchanger in which circulated heat carrier medium that absorbs heat in the fuel cell stack releases said absorbed heat energy to an ambient air stream conducted through the heat exchanger, the latter being referred to in the present case in generalized form as a cooler structure. Air-cooled fuel cell stacks are, however, also known, which themselves form or comprise a suitable cooler structure past which or through which an ambient air stream is conducted for cooling purposes; in this regard, confer for example the German patent application with the file reference 10 2012 206 459.1, which reference does not constitute a prior publication. In most cases, for the delivery of the ambient air stream through the cooler structure—be it an independent heat exchanger or a cooler structure integrated in the fuel cell stack—an independent delivery device (blower, fan, etc.) is required which, in part, has a relatively high energy requirement.
A problem addressed by the present invention is that of specifying an improvement in this regard.
The solution to the problem consists in an exhaust-air guide of a fuel cell stack, in particular in a motor vehicle, having a cooling device, which cooling device belongs to the functional environment of the fuel cell stack and which is in the form of a cooler structure through which ambient air flows. The exhaust air of the fuel cell stack is guided to a point upstream of the cooler structure such that the exhaust air flows through the cooler structure in a throughflow direction and, in so doing, entrains ambient air in accordance with the jet pump principle.
According to the invention, the exhaust air of the fuel cell stack—in the case of PEM fuel cells, this is the cathode exhaust-air stream—is at least partially conducted through the cooler structure, wherein the pressure or positive pressure of the exhaust-air stream is utilized for the delivery of ambient air (as cooling air). According to the invention, for this purpose, the exhaust-air stream (or at least a part of the same) is guided to the cooler structure such that, as it enters the cooler structure and subsequently flows through the same, it draws or entrains further ambient air into the cooler structure. In this respect, the present invention utilizes the fundamentally known jet pump principle (known, in particular, from a suction jet pump formed by a suitable pipe junction). It is expressly pointed out here that, at a cooler structure, it is by no means necessary for the fuel cell exhaust air and the ambient air to be conducted in a pipe junction to a point upstream of the cooler structure, or into the cooler structure; rather, it is adequate for the exhaust air of the fuel cell stack to be suitably guided to a point upstream of the cooler structure, for example in multiple pipes that are oriented at least approximately parallel to the inflow surface of the cooler structure, from which pipes the exhaust air emerges via outlet openings in the wall of the pipes (“pipe wall”). The outlet openings are situated at a suitable angle with respect to the throughflow direction of the (desired) cooling air stream through the cooler structure. It is always possible in the proposed way for the pressure potential that exists in the exhaust-air stream of the fuel cell stack to be utilized for the delivery of ambient air and thus of cooling air through the cooler structure.
As regards the fuel cell exhaust air or the exhaust-air stream, the pressure potential or positive pressure thereof results from the prior delivery of ambient air, as reaction air, into the fuel cell stack. The ambient air reacts there in a known manner at one of the electrodes—in the case of a PEM fuel cell, at the cathode—with the fuel stream (in particular in the form of hydrogen) that is guided to the other side of the respective cathode-electrolyte-anode unit, before the ambient air is subsequently discharged from the fuel cell stack as an exhaust-air stream. Normally, the temperature and, in particular, the moisture content of the fuel cell stack exhaust air is elevated in relation to the ambient air that is initially supplied as reaction air to the fuel cells. In one advantageous refinement of the invention, it may therefore be provided that, according to the invention, before being guided to the cooler structure, the exhaust air of the fuel cell stack is cooled in a suitable heat exchanger (preferably again with the aid of ambient air), with moisture advantageously being condensed out, such that the exhaust-air stream of the fuel cell stack does not give rise to mist formation, such as is otherwise normally observed.
In one advantageous refinement of the present invention, the inflow direction and/or the inflow speed of the fuel cell exhaust air relative to the cooler structure, and thus for example the outflow direction and/or the outflow speed of the exhaust air from the pipes and generally from any system via which the fuel cell exhaust air is guided, in the manner according to the invention, to a point upstream of the cooler structure, may be variable in targeted fashion. For example, for this purpose, the outlet openings in the pipe wall of the pipes may be variable by use of a slide or similar. For example, by way of slides, the cross-sectional area of the outlet openings can be varied, giving rise to different outflow speeds for the fuel cell exhaust-air stream; it is however also possible by way of slides or the like for the inflow angle at which the fuel cell exhaust-air stream impinges on the cooler structure to be varied. In the case of the pipes, mentioned by way of example, through which the fuel cell exhaust air is guided to a point upstream of the cooler structure, it is however also possible for the pipes themselves to be rotated about their longitudinal axis, and thus for the inflow direction of the exhaust-air stream relative to the cooler structure to be varied. If multiple pipes are provided through which the fuel cell exhaust air is conducted to a point upstream of the cooler structure, it is also possible, in a manner dependent on a wide variety of boundary conditions, for some of the pipes to be deactivated, so to speak, that is to say not charged with fuel cell exhaust air, whereby the entire exhaust-air stream is distributed over a smaller number of pipes and thus the flow speed of the exhaust-air stream in the smaller number of pipes is increased.
At this juncture, it is expressly pointed out that it is by no means necessary, as is optionally proposed, for the fuel cell exhaust air to be guided to the air inflow side of the cooler structure via multiple pipes with outlet openings provided in the pipe wall; rather, it is also possible for other air guidance systems to be used for this purpose.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.